1. Field of the Invention
The present invention relates to a force sensor circuit having a force sensor element for detecting, through the use of a piezoelectric member, a physical quantity such as force, acceleration, or magnetic force acting from outside.
2. Description of the Related Art
There has been an increasing demand for sensors capable of accurately detecting a physical quantity such as force, acceleration, or magnetic force in the fields of automobiles and mechanical industries (such a sensor will be hereinbelow referred to as xe2x80x9ca force sensorxe2x80x9d). For example, a sensor in which a plurality of piezoelectric members are disposed on a flexible plate having an operating member thereon, has been disclosed (Japanese Unexamined Patent Publication No. 5-26744).
This sensor is constituted so that the flexible plate is bent by a force corresponding to a physical quantity acting on the operating member from outside. The sensor can three-dimensionally detect the direction and the magnitude of a physical quantity by detecting, with a single sensor element, electric charges generated in piezoelectric members in accordance with bending of the flexible plate.
An acceleration sensor using an operating member as a weight will now be described below by way of example. As illustrated in FIG. 2, when an acceleration xe2x80x9caxe2x80x9d acts on the sensor from outside, an inertia force xe2x80x9cfxe2x80x9d acts on a weight 10 in the reverse direction of that of the acceleration, whereby a flexible plate 12 suspended between supporting bases 11 is bent at its portions between the weight 10 and the respective supporting bases, by the effect of the inertia force xe2x80x9cfxe2x80x9d.
Electric charges in accordance with the direction and magnitude of the bending are generated in piezoelectric members 13 disposed on the flexible plate 12. This allows the direction and magnitude of acceleration acting from outside to be detected by extracting the electric charges from upper electrodes 14x, 14y, and 14z, and lower electrodes 18 as electric signals.
As shown in FIG. 3(a), the above-described force sensor element is typically constructed as a force sensor circuit 21 comprising at least a force sensor 22, a resistor 23 for converting electric charges generated in the piezoelectric members into voltages, and an operational amplifier (OP amp.) 24 for amplifying the voltages, which are all disposed on a printed circuit board 25.
More specifically, as shown in FIG. 3(c), since voltages are generated between a set of electrodes of the resistor 23 based on electric charges generated in the piezoelectric members 26 of the force sensor element, these voltages are input to the operational amplifier 24 for amplification.
It is well known that, in a force sensor as shown in FIG. 3(a), there is a frequency range peculiar to a sensor circuit that the sensor circuit can output, as shown in FIG. 4 (the lower limit of detection fLC and the upper limit of detection: fHC; the frequency range will be referred to as xe2x80x9cfrequency characteristicsxe2x80x9d hereinbelow), and that the lower limit of detection fLC is determined by the capacitance C of the piezoelectric element 26 and the resistance value R of the resistor 23 (see the equation (1) shown below). That is, even if a given physical quantity acts on a force sensor element, the detection sensitivity of the force sensor is very low in the case where an operating member vibrates within a frequency range below the fLc value or above the fHC value.
fLC=1/(2xcfx80RC)xe2x80x83xe2x80x83(1)
Therefore, if the force sensor is required to detect an ultra low frequency vibration, it becomes necessary for the fLC value to be reduced by designing the capacitance C of piezoelectric element and/or the resistance R of the resistor to have high values.
The above-described force sensor element has, however, a size as small as about 5xc3x975xc3x971.5 mm, and each of the piezoelectric members composing piezoelectric element also has a small area, so that it is unlikely that the capacitance C can be designed to have a high value. In order to reduce the fLC value, therefore, the resistance R shown in the equation (1) is required to be designed to have a high value.
Accordingly, in the force sensor circuit as shown in FIG. 3(a), a resistor having a high resistance value of 10 Mxcexa9 or more (such a resistor is hereinbelow referred to as xe2x80x9can ultra-high-resistance resistorxe2x80x9d) is used as a resistor for converting electric charges generated in piezoelectric members into voltages.
There is a problem, however, that in the force sensor circuit using an ultra-high-resistance resistor as shown in FIG. 3(a) the resistance value R of the ultra-high-resistance resistor varies in the temperature characteristic among all sensor circuits, resulting in variances in the fLC value (i.e., frequency characteristic) among all the sensor circuits, as shown in FIG. 12(a).
The present invention has been achieved to overcome the above-described problem of the prior art and aims to provide a force sensor circuit capable of eliminating any variance in the fLC value (i.e., frequency characteristics) among all sensor circuits, and allowing the frequency characteristics of all the sensor circuits to be equalized, in the force sensor circuit having at least one ultra-high-resistance resistor of at least 10 Mxcexa9.
In accordance with the present invention, there is provided a force sensor circuit comprising: a force sensor element having an operating member, a supporting base having a hollow portion and disposed around in the vicinity of the operating member, a flexible plate extending across over the hollow portion of the supporting base so as to suspend the operating member, and at least one piezoelectric element having a piezoelectric member sandwiched between a set of electrodes; at least one resistor having a resistance value of at least 10 Mxcexa9 for converting an electric charge generated in a piezoelectric member of the piezoelectric element into a voltage; and an operational amplifier for amplifying the voltage generated between the set of electrodes of the resistor, wherein the resistor is formed on the upper surface of the supporting base or on the upper surface of the operating member of the force sensor element.
A force sensor circuit of the present invention may be either a so-called two-axial force sensor element having piezoelectric elements corresponding to two arbitrary orthogonal axes of the x, y, and z axes, or a so-called three-axial force sensor element having piezoelectric elements corresponding to three orthogonal axes of x, y, and z. Also, this force sensor may be an acceleration sensor using an operating member as a weight.
It is preferable that a force sensor circuit of the present invention is constructed by electrically connecting a force sensor with a printed circuit board by wire bonding or flip-chip bonding.
The above and other objects, features and advantages of the present invention will become clear from the following description of the preferred embodiment when the same is read in conjunction with the accompanying drawings.